Gratings are embedded in laser diode structures to stabilize the lasing wavelength. The distributed Bragg reflector (DBR) grating laser devices have gratings outside of the gain (electrically pumped) region while distributed feedback (DFB) grating devices have a grating within the gain region. In the conventional process, a grating is created in an exposed p-doped cladding layer of a semiconductor wafer. The cladding layer is coated with photoresist and exposed to the interference pattern from two plane waves of ultra-violet or blue light, which produce the desired diffraction pattern on the resist. The perodicity of the interference pattern can be precisely adjusted by setting the angles of incidence of the two beams. The photoresist is developed and etchants are applied to transfer the grating that was exposed in the photoresist into the surface of the cladding layer. If the grating is to be in the pumped region, epitaxial layers are regrown over the grating. If the grating is to lie outside of the pumped region, the overgrowth is unecessary and a simple insulating layer of SiO2 can be used to protect the surface. After the grating and other wafer processing is completed, the wafer bar must be cleaved to provide mirror facets that define the Fabry-Perot cavity.
Cleaving is performed by mechanically scribing the wafer with a precision instrument. Such instruments can only locate the scribe line to within ˜10 microns. However, the actual location of the facet is determined by the crystal plane of the wafer which is closest to the scribe line. By comparison, the light wavelength λ inside the laser is less than one micron. For example, a 808 nm, diode laser with an index of refraction of ˜3.3 has a wavelength inside the diode of ˜240 nm (0.24 microns). Thus, the distance between the cleaved facet and the grating grooves is essentially a random number of waves. The uncontrolled phase between the grating and facet leads to unwanted spectral variation from laser to laser.
For a grating to stabilize the wavelength λ of emitted light, the lines of the grating should be separated from each other by a distance Λ=λ/n, where n is the cladding layer's index of refraction, n. For example, if λ=880 nm, Λ=880/3.3 or only about 240 nm. The cleaved facet should be located to within a small fraction of Λ from the first grating line, say 10% of Λ. A larger error in the “z” axis position of the cleaved facet may cause a reflection that will make the grating's effectiveness less than optimal. If the facet is cleaved so that the spacing between the grating and the cleaved facet is at an odd multiple of the half wavelength Λ/2, the grating will not stabilize the laser bar's light wavelength. The uncontrolled phase between the grating and facet will lead to unwanted spectral variations and therefor lower the effective yield of lasers cleaved from the wafer. Low yields are especially unacceptable in the manufacture of high-power laser bars.